basic/terminal-util: introduce SYSTEMD_COLORS environment variable

... to determine if color output should be enabled. If the variable is not set,
fall back to using on_tty(). Also, rewrite existing code to use
colors_enabled() where appropriate.

man/systemd.xml

@@ -834,6 +834,13 @@
         script runlevel link farms.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>$SYSTEMD_COLORS</varname></term>
+
+        <listitem><para>Controls whether colorized output should be generated.
+        </para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>$LISTEN_PID</varname></term>
         <term><varname>$LISTEN_FDS</varname></term>

src/basic/terminal-util.c

@@ -1135,3 +1135,16 @@ int open_terminal_in_namespace(pid_t pid, const char *name, int mode) {
 
         return receive_one_fd(pair[0], 0);
 }
+
+bool colors_enabled(void) {
+        const char *colors;
+
+        colors = getenv("SYSTEMD_COLORS");
+        if (!colors) {
+                if (streq_ptr(getenv("TERM"), "dumb"))
+                        return false;
+                return on_tty();
+        }
+
+        return parse_boolean(colors) != 0;
+}

src/basic/terminal-util.h

@@ -79,37 +79,38 @@ unsigned lines(void);
 void columns_lines_cache_reset(int _unused_ signum);
 
 bool on_tty(void);
+bool colors_enabled(void);
 
 static inline const char *ansi_underline(void) {
-        return on_tty() ? ANSI_UNDERLINE : "";
+        return colors_enabled() ? ANSI_UNDERLINE : "";
 }
 
 static inline const char *ansi_highlight(void) {
-        return on_tty() ? ANSI_HIGHLIGHT : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT : "";
 }
 
 static inline const char *ansi_highlight_underline(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_UNDERLINE : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_UNDERLINE : "";
 }
 
 static inline const char *ansi_highlight_red(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_RED : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_RED : "";
 }
 
 static inline const char *ansi_highlight_green(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_GREEN : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_GREEN : "";
 }
 
 static inline const char *ansi_highlight_yellow(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_YELLOW : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_YELLOW : "";
 }
 
 static inline const char *ansi_highlight_blue(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_BLUE : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_BLUE : "";
 }
 
 static inline const char *ansi_normal(void) {
-        return on_tty() ? ANSI_NORMAL : "";
+        return colors_enabled() ? ANSI_NORMAL : "";
 }
 
 int get_ctty_devnr(pid_t pid, dev_t *d);

src/journal/journalctl.c

@@ -2336,7 +2336,7 @@ int main(int argc, char *argv[]) {
                         flags =
                                 arg_all * OUTPUT_SHOW_ALL |
                                 arg_full * OUTPUT_FULL_WIDTH |
-                                on_tty() * OUTPUT_COLOR |
+                                colors_enabled() * OUTPUT_COLOR |
                                 arg_catalog * OUTPUT_CATALOG |
                                 arg_utc * OUTPUT_UTC;
 

src/login/loginctl.c

@@ -88,7 +88,7 @@ static OutputFlags get_output_flags(void) {
                 arg_all * OUTPUT_SHOW_ALL |
                 arg_full * OUTPUT_FULL_WIDTH |
                 (!on_tty() || pager_have()) * OUTPUT_FULL_WIDTH |
-                on_tty() * OUTPUT_COLOR;
+                colors_enabled() * OUTPUT_COLOR;
 }
 
 static int list_sessions(int argc, char *argv[], void *userdata) {

src/machine/machinectl.c

@@ -108,7 +108,7 @@ static OutputFlags get_output_flags(void) {
                 arg_all * OUTPUT_SHOW_ALL |
                 arg_full * OUTPUT_FULL_WIDTH |
                 (!on_tty() || pager_have()) * OUTPUT_FULL_WIDTH |
-                on_tty() * OUTPUT_COLOR |
+                colors_enabled() * OUTPUT_COLOR |
                 !arg_quiet * OUTPUT_WARN_CUTOFF;
 }
 

src/systemctl/systemctl.c

@@ -247,7 +247,7 @@ static OutputFlags get_output_flags(void) {
                 arg_all * OUTPUT_SHOW_ALL |
                 arg_full * OUTPUT_FULL_WIDTH |
                 (!on_tty() || pager_have()) * OUTPUT_FULL_WIDTH |
-                on_tty() * OUTPUT_COLOR |
+                colors_enabled() * OUTPUT_COLOR |
                 !arg_quiet * OUTPUT_WARN_CUTOFF;
 }